Piezoelectric transducer



y 1 1958 J. w. CROWNOVER 2,836,737

PIEZOELECTRIC TRANSDUCER Filed July 20, 1953 FIG. I. I0

I ls L-a I FIG. 4.

lBa

I INVENTOR 9 3O JOSEPH w. CROWNOVER 3| 32 BY- ATTORNEYS United StatesPatent t 35 2,836,737 PIEZOELECTRIC TRANSDUCER Joseph W. Crownover,Sherman Oaks, Calih, assignor, by mesne assignments, to ElectricMachinery Mfg.

Company, a corporation of Minnesota Application July 20, 1953, SerialNo. 369,123 11 Claims. (Cl. Mil-81) This invention relates to improvedelectromechanical transducers and more particularly to an improvedbender type transducer characterized by its ability to produce underapplied voltage a greater bending displacement or deflection per unitlength of the bender element than has heretofore been obtainable.

Bender elements of the type disclosed herein have great utility in thoseapplications wherein there is need for a mechanism to produce smallrapid linear or substantially linear movements, in a reciprocal sense,with or without a high rate of repetition, or frequency of motion. Forexample, certain types of relays require actuator means for opening andclosing electrical contacts which are spaced only a short distanceapart. A motor element comprising joined, elongated strips ofelectrostrictive titanate material that is worked in bending by avoltage applied through one of the strips so as to actuate a relay unitis disclosed and claimed in my copcnding application, Serial No.357,132, filed May 25, 1953, and entitled Piezoelectric Relay, nowabandoned.

it is known that certain bonded titanate composi liOl'lS such as bariumtitanate, or strontium titanate, or mixtures of the two areelectrostrictive in nature, that is to say, a proper voltage appliedacross a portion of such a material will bring about an expansion of thematerial in the direction of the applied voltage and a correspondingcontraction of the material in a plane lying at right angles to thedirection of the applied voltage. The change in dimension of thematerial in the direction of the applied voltage is roughly two andone-half times greater than the oppositely directed change in dimensionof the material in the plane at. right angles to the voltage gradient.Furthermore, it is known that ceramic materials of this type whensubjected to voltage gradients become polarized, or made relativelypermanently piezoelectric, if the temperatures thereof are kept belowthe Curie point for the material. As the material is heated to the Curietemperature, rcrnanent polarization in the material diminishes to zero,and the piezoelectric properties thereof are lost. The Curie point forsuch a material occurs when the dielectric constant thereof reaches amaximum as the temperature is increased.

fit/hen ceramic materials of the type referred to are once positivelypolarized, they may be made to expand in the direction of the appliedvoltage gradient by applying a positive voltage across the material inthe direction of the polarizing gradient; similarly, the material may bemade to contract in the direction of the applied voltage by applying anegative voltage thereto. However, once the material has been polarized,the displacement from the polarized position resulting from theapplication of a positive voltage across the material equal to thepolarizing voltage gradient, is much less than the maximum displacementfrom the unpolarized position obtained during initial polarization.Furthermore, a negative voltage gradient of not inconsiderable magnitudeis required before the material can be caused to contract to the extentthat it will again assume its unpolarized dimension. If a greaternegative voltage is applied to the material after it has once contractedto its unpolarized dimension, the material will begin to expand again tobecome oppositely polarized And correspondingly, the material must besubjected to a considerable positive voltage to depolarizc it. Thus itis evident that for many applications, the displacements obtainable fromworking the material in its polarized state are too small for practicaluse.

it is also known that the material referred to becomes polarized withthe application thereto of voltage of any magnitude; however, the timerequired to completely polarize the material will be increased if weakervoltages are applied across the material. For example, if bariumtitanate is subjected to a voltage of 60 volts per mil thickness of thematerial, the time required to completely piezoelcctrically sensitizethe material will be approximately 4 minutes, whereas it the voltage isdecreased to 30 volts per mil thickness of the material the timerequired is increased to approximately 40 minutes. If the voltageapplied across the material is increased to volts per mil, which isroughly the dielectric breakdown value for barium titanate, the timerequired to piezoelectrically sensitize the material will be decreasedto only a few seconds. Since many applications call for a frequency ofreciprocal movement considerably in excess of several times per second,it is evident that even with the cyclic application of an actuatingvoltage of 100 volts per mil, the material could not attain its fullpiezoelectric sensitivity, or stated more simply, the material could notbe made to move back and forth with the maximum amplitude obtainableunless the cyclic period were of the order of 4 times the time requiredto completely sensitize the material. Thus it is evident that there is:a need for a device that will make use of the small reciprocalmovements that are obtainable as a result of applying voltages to thepolarized, piezoelectric material, and to amplify these small movementsto practical, usable values.

if the material referred to is operated at temperatures immediatelyabove the Curie point, it is known that the remanent piezoelectriceffect is lost, that is to say, the ability of the material to retaininduced polarization is lost, which necessarily results in the loss ofthe ability of the material to be made to expand and to contract fromits polarized state. However, the material still retains the ability toexpand in the direction of the applied voltage, and the extent of theexpansion is approximately the same as exists below the Curie point aslong as the operating temperature is not increased too far beyond theCurie temperature. By way of explanation, it is found that attemperatures below the Curie point for the material the degree to whichthe electrostrictive titanate responds to an applied voltage, asmeasured by the coupling coeificient thereof, remains substantiallyconstant; however, as the operating temperature is increased above theCurie point, the degree to which the electrostrictive material respondsto an applied voltage diminishes slowly to zero.

With the removal of the applied voltage, the material will approximatelyresume its initial unexpanded dimension, at temperatures above the Curiepoint, so that negative voltages are not required to cause the materialto resume its original dimension. Also, the material may be caused to befully expanded with the application of a voltage in a much shorter time.For example, when a voltage of 100 volts per mil is applied to apolycrystalline barium titanate material at temperatures above the Curiepoint for the material, it will become fully expanded in approximately10 milliseconds, and furthermore, when the voltage is removed, thematerial will resume its original dimension in approximately the sameshort interval of time.

As far as available electrostrictive materials themselves are concerned,the principal disadvantage accompanying the use of pure barium titanateabove the Curie point consists in the relatively high temperature atwhich the Curie point exists, it being in the neighborhood of PatentedMay 27, 1958' to 130 degrees centigrade. Thus pure barium titanateceramic can only be practically utilized in this way if the material isartificially heated. The principal disadvantage attending the use ofpure strontium titanate stems from the fact that the Curie pointtemperature thereof is so low, being less than minus 100 degreescentigrade, that at ordinary temperatures the coupling coefficient ordegree to which the material will respond electrostrictively to anapplied voltage is considerably reduced, resulting in the production ofrelatively small displacements. However, the Curie point temperaturesfor mixtures of barium titanate and strontium titanate lie between theextreme temperatures for the pure materials. Therefore, an ideal mixtureof the two materials will be such that the Curie point temperaturethereof lies just below the minimum expected operating temperature so asto gain the benefits of the relatively large and rapid displacements ofthe material which are available above the Curie point temperature whenthe material is worked electrostrictively, and so as to minimize theadverse effect upon the coupling coefficient of the material resultingfrom operating the material at temperatures considerably above the Curietemperature.

For example, if we assume the minimum temperature of the operating rangeto be 22 centigrade, a material comprising, by weight, 73% bariumtitanate and 27% strontium titanate may be chosen since this has a Curiepoint of approximately 20 centigrade.

With these principles and limitations in mind, it is, accordingly, theprincipal object of my invention to produce an improved electrostrictivebending transducer wherein means is provided for causing the transducerto produce, under an applied voltage, a deflection per unit length ofthe transducer which is considerably greater than has heretofore beenobtainable.

It is another object of my invention to provide an improved titanatetransducer wherein there is provided a means for causing the transducerto produce, under applied positive or negative voltage, positive ornegative deflections having magnitudes considerably greater than haveheretofore been obtainable.

It is another object of my invention to provide an improvedelectrostrictive titanate bending transducer wherein there is provided ameans for causing the transducer to produce, under rapid cyclicapplications of voltage, amplitudes of deflection having magnitudesgreater than have heretofore been producible.

It is a further object of my invention to provide an improvedelectrostrictive titanate bending transducer wherein means is providedfor obtaining and combining high mechanical compliance, favorablebending strength, and a high degree of displacement per unit length ofthe bending elements.

These and other objects and advantages of the present invention willbecome apparent from a consideration of the following description andthe appended claims in conjunction with the accompanying drawingswherein:

Fig. l is a cross sectional View of a bender type transducer of thepresent invention;

Fig. 2 is a View of the bender unit taken on line 2-2 of Fig. 1;

Fig. 3 is an enlarged cross sectional view of the bender unit taken online 33 of Fig. 1;

Fig. 4 is an enlarged fragmentary sectional view of a portion of thebender unit and is taken on line 44 of Fig. 3;

Fig. 5 is a cross sectional view of a modified form of the bender unitof the present invention;

Fig. 6 is an enlarged fragmentary sectional view of a portion of thebender unit illustrated in Fig. 5;

Fig. 7 is a cross sectional view of another form of the bender unit ofthe present invention;

Fig. 8 is an enlarged cross sectional view of the bender unitillustrated in Fig. and is taken on line 38 thereof; and

Fig. 9 is a cross sectionalview of a portion of the bender unitillustrated in Fig. 7 and is taken on line 9-9 thereof.

Referring now to the cantilevered bending transducer unit illustrated inFig. 1, it will be seen that a pair of elongated, thin strips orelements of ceramic material 11 and 12 are firmly joined together andinsulated from one another by applying a cement 13 such as anonconducting thermosetting resin between adjacent faces of the strips.The strips thus adjacent one another in face to face relatt n. Each ofthe strips is longer than it is wide, and wider than it is thick so asto give the bender unit the greatest degree of mechanical compliance ina plane defined by the length and thickness dimensions of the strips.The strips are composed of a polycrystalline aggregate such as atitanatc ceramic which is electrostrictive in nature. The preferredcomposition is barium titanate ceramic, or strontium titanate ceramic,or mixtures thereof.

Electrostrictive element 11 has a pair of electrodes 14 and 15 formed onopposite faces thereof, as by applying silver paint thereto.Electrostrictive element 12 has a plurality of transverse comborgridtype electrodes 18a and 18b formed on face 19 thereof, as byapplying silver paint thereto. The transverse electrodes 18a and 18bextend spanwise across a portion of the face 19 of the element 12 andare closely spaced to one another. The transverse electrodes 18a areinterconnected by means of an elongated conducting band 21 runninglengthwise along one side of face 19, and the transverse electrodes 18bare interconnected by means of an elongated conducting band 22 runninglengthwise along the opposite side of face 19. A source of electricpotential 23 is connected across electrodes 14 and 15, and acrossconducting bands 21 and 22, so

as to apply a voltage between electrodes 14 and 15, and between adjacenttransverse electrodes 18a and 18b. The major component of the voltagegradient between electrodes 14 and 15 will lie parallel to the thicknessdimension of element 11, and the major component of the voltage gradientbetween any two adjacent transverse electrodes 18a and 1812 will lieparallel to the length dimension of strip 12. schematically shownvoltage gradients 23 and 24 extending betweenadjacent pairs oftransverse electrodes 18a and 18b will be oppositely directed, asillustrated in Fig. 4. It. is desirable to minimize the width of thesegments between electrodes 18a and 1812 so as to maximize thelongitudinal voltage gradients in strip 12. Thus there is provided ameans for establishing a first voltage gradient or potential differenceparallel to the thickness dimension of one of the elongatedelectrostrictive strips, and likewise a series of voltage gradientshaving major components directed parallel to the length dimension of thesecond elongated electrostrictive element.

In accordance with the principles outlined in a previous portion of thisdescription, when voltage is applied through strip 11 parallel to thethickness dimension thereof, the thickness of the strip will increaseand the length thereof decrease. Similarly, a voltage gradient appliedthrough strip 12 parallel to the length dimension thereof will cause thelength of the strip to increase and the thickness thereof to decrease.Stated another way, the result of the application of voltage to thevarious electrodes will be a contraction in the length mode of strip 11,and an elongation in the length mode of strip 12. Since the strips arefirmy fastened together, the elongation of strip 12 will be resisted bythe contraction of strip 11, so as to yield a bending of the free end ofthe cantilevered transducer unit upwardly, or toward the contractingside of the transducer unit. Furthermore, the degree of bending, oramount of deflection per unit length of the bending unit is considerablyin excess of the deflection that would result were only one of theelongated strips caused to elongate or contract against the reactionprovided by an adjacent unactivated strip. Thus the free end of thecantilevered bending unit may be caused to undergo a maximum deflection,and thereby to do useful work in many applications whereinelectrostrictive bender type transducers were heretofore impractical dueto the limited displacements obtainable therefrom.

It is possible to construct a bender type transducer having a pair ofjoined, elongated strips with a common electrode between the strips anda pair of electrodes on opposite faces of the strips, so that a voltagegradient may be applied across each strip between the common electrodeand the outer electrodes. If the strips are operated above the Curiepoint temperature, there will be no bending produced, since both stripswill expand in the direction of the voltage gradient and contracttogether in the longitudinal mode. However, if the strips have beenpolarized previously, the first in the direction of the presentlyapplied voltage gradient, and the second in the direction opposite tothe applied voltage gradient, and if they are operated below the Curiepoint temperature, then the first strip will expand in the longitudinalmode and the second strip will contract in the longitudinal mode toproduce bending. However,

the amount of bending thereby producible is severely limited due to thefact that if the voltage gradients are increased above approximatelyvolts per mil, the second strip will cease to contract in thelongitudinal mode and will begin to expand along with the first strip sothat further bending will be precluded. In contradistinction to thistype of bender unit, one great advantage of the present invention is thefact that voltages up to the breakdown strength of the ceramic materialmay be applied to the material, and bending will increase so long as thevoltage gradient is increased. Thus in the present invention, bendingdoes not cease at the 10 volt per mil voltage gradient level, butcontinues as long as the applied voltage gradient is increased.

The bender unit illustrated in Figs. 5 and 6 is the same as thatillustrated in Fig. l with the addition thereto of a second series oftransverse electrodes a and 25b formed on the face of strip 12, oppositeface 19 thereof. Electrodes 25a and 25b are spaced longitudinally of theelement and alternately disposed, similar to electrodes 18a and 18b andfurthermore, electrodes 25a are spaced opposite electrodes 18a andelectrodes 25b are spaced opposite electrodes 18b. Electrodes 25a areinterconnected by means of a conductive band, not shown, and areelectrically connected to electrodes 18a. Similarly, electrodes 2% areinterconnected by means of a conducting band, and are electricallyconnected to electrodes 1811. When a voltage is applied acrosselectrodes 18a, 18b, 25b and 25a, a stronger voltage gradient componentparallel to the length dimension of strip 12 is established, asschematically illustrated in Fig. 6, thereby increasing the tendency ofthe electrostrictive strip 12 to expand in the lengthy mode, as comparedwith the strip having transverse electrodes applied to'one side onlythereof. Thus, a somewhat stronger moment of force is set up in strip12, opposing the oppositely directed moment in strip 11, and yielding asomewhat increased deflection of the free end of the bending unit 10 inthe upward direction.

The bender unit illustrated in Figs. 7, 8, and 9 is similar to the unitillustrated in Fig. l, with the addition thereto of an elongated thinstrip 3% of a metal conductor such as a strip of brass. Also, the strips11 and 12 are not insulated from one another. in shape, there being anelongated blanked-out portion 31 formed at the center thereof, which isperimetrically enclosed by longitudinal side strips 32 and transverseend strips 33. The strip is disposed between elongated ceramic strips 11and 12' in alignment therewith,

Strip 30 is perimetrical 6 conductive cement or fastening means notshown. Strip 30 is only a few thousandths of an inch thick so as toadmit of a high degree of bending compliance. The strip 30 is inelectrical contact with electrode 15 on one face of strip 11, and withconducting band 21 on strip 12 to which the transverse electrodes 18aare joined.

Band 21 is connected to metal strip 30 by means of a conducting band 36of silver paint applied to the side wall 35 of ceramic strip 12. Apotential difference may be applied between electrodes 14 and 15, andbetween transverse electrodes 18a and 18b by applying the potentialbetween conducting strip 30 and electrode 14, and similarly betweenstrip 30 and electrodes 18b, since strip 30 is electrically joined toelectrodes 15 and 18a. As a result, strip 11 will contract in itslongitudinal mode and strip 12 will expand in its longitudinal mode sothat the free end 38 of the transducer unit will be displaced upwardlytoward the contracting side of the transducer unit. By limiting theaerial extent of metallic conducting strip 30 to two narrow transverseend strips 33 and two narrow side strips 32, interference with thevoltage gradients between adjacent transverse electrodes 18a and 1812caused by the proximity thereto of the charged side portions 33 of metalstrip 30 will be minimized. Thus there is provided a bending transducerhaving greatly increased bending strength and yet having a high degreeof mechanical compliance in the plane of bending.

It is pointed out that if the ceramic strips 11 and 12 are composed ofbarium titanate, they may be polarized by means of an applied voltageand operated piezoelectrically at ordinary temperatures to yieldpositive and negative bending deflections with respect to the polarizedposition of the transducer. Even though the piezoelectrically induceddeflection in each strip is relatively small by itself, the particularcombination of bending elements as disclosed will result in relativelylarge bending deflections per unit length of the transducer unit 10, dueto the novel combination of ceramic strips 11 and 12 and electrodes 18aand 18b.

Also, if the ceramic strips 11 and 12 are worked at a temperatureslightly above the Curie point, which can readily be done for ordinaryapplications by utilizing ceramic strips composed of barium titanate andstrontium titanate in such relative proportions that the Curie pointwill be just below the lower end of the operating temperature range, thebending deflections obtained per unit length of the transducer unit willbe to all intents and purposes maximized, and relative rapidity ofdeflection will also be realized, to the end that the bending transducermay be advantageously utilized as an actuator or motor unit in manyapplications requiring small, rapid actuating displacements.

While I have described my invention with particular reference to bendertype transducers having a pair of elements formed of bonded titanatecompositions such as barium titanate, or strontium titanate, or mixturesof the two, having electrostrictive or piezoelectric characteristics,because such compositions are eminently suited for commercialapplications, in its broader aspects the invention is also applicable toa pair of elements formed of some other electromechanically sensitivedielectric material, such as a portion of a single crystal of apiezoelectric substance or an aggregate or composition of crystals otherthan titanates, such as certain barium zirconates. 1' thereforecontemplate that various changes and modifications can be made withoutdeparting from the invention as indicated by the claims which follow.

I claim:

1. In a bender type transducer: a pair of elongated electrostrictivedielectric strips, each of said strips having a pair of major facesdefined by the length and width dimensions thereof, said strips beingrigidly joined together and insulated from one another; a pair of spacedelectrodes each one of which is disposed longitudinally and is securedin position by means of an electrically in intimate physical contactwithone of the major faces "Z of one of said elongated strips; means forestablishing a potential difference between said electrodes; a pluralityof electrodes disposed in intimate physical contact with one of themajor faces of the other of said strips, said electrodes lyingtransversely to the length axis of said strip and being spaced from oneanother; and means for establishing a potential difference betweenadjacent transversely lying electrodes.

2. in a bender type transducer: a pair of elongated electrostrictivedielectric elements, said elements being joined together and insulatedfrom one another; first and second electrodes disposed in intimatephysical contact with an opposite pair of faces of one of said elements;a plurality of electrodes disposed in intimate physical contact with atleast one of the faces of the other of said elements, said electrodeslying transversely to the length axis of said element and being spacedfrom one another; and means for establishing a potential differencebetween said first and second electrodes and between'adjacent transverseelectrodes.

3. In a bender type transducer: a pair of electrostrictive dielectricelements, said elements being joinedtogether and insulated from oneanother; electrically chargeable electrode means for establishing avoltage gradient through a portion of one of said elements having acomponent extending parallel to the length dimension of said element;the other of said elements having a pair of major faces, first andsecond electrodes disposed respectively in physical contact with saidmajor faces; and means for establishing a voltage gradient between saidfirst and second electrodes.

4. in a bender type transducer: :1 pair of electrostrictive dielectricelements, each of said elements having a pair of major faces, saidelements being joined together in face to face relation and insulatedfrom one another; means for establishing a first voltage gradientthrough one of said elements, having its major component directedparallel to the thickness dimension of said element, said meansincluding a pair of electrodes formed on opposite faces of said one ofsaid elements; and electrically chargeable electrode means forsimultaneously establishing a second voltage gradient extending througha portion of the other of said elements, having its major componentextending parallel to the length dimension of said other element.

5. In a bender type transducer: a pair of elongated elements, saidelements being joined together in face to face relation; electricallychargeable electrode means for establishing a first voltage gradientthrough one of said elements, having its major component directedparallel to the thickness dimension of said element; and means forestablishing a second voltage gradient through a portion of the other ofsaid elements, having its major component parallel to the lengthdimension of said other element.

6. ln abender type transducer: a plurality of elongated electrostrictiveelements, said elements being joined together in face to face relation,said elements being insulated from one another; electrically chargeableelectrode means for establishing a first voltage gradient through one ofsaid elements, having its major component directed parallel to thethickness dimension of said element; and means for establishing a secondvoltage gradient tirough a portion of the other of said elements, havingits major component directed parallel to the length dimension of saidsecond element.

7. In a bender type transducer: a'plurality of elongated strips joinedtogether in face to face relation, said strips including electricallyconductive metallic strip, a first electrostrictive titanate stripjoined to one face of said metallic strip, and a second electrostrictivetitanate strip joined to the opposite face of said metallic strip; apair of spaced electrodes each of which is disposed in intimate physicalcontact with one of the opposite major faces of the first of saidtitanate strips; means for establishing a potential difference betweensaid electrodes; a plurality of electrodes disposedin physical contactwith one of the major faces of the second of said titanate strips, saidelectrodes lying transversely to the length axis of said strip and beingspaced from one another; andmeans for establishingfa potentialdifference between said transversely lying electrodes.

8. In a bender type transducer: a plurality of elongated strips joinedtogether in face to face relation, said strips including anelectricallyconductive metallic strip, a first electrostrictive titanate stripjoined to one face of said metallic strip, and a second electrostrictivetitanate strip joined to the opposite face of said metallic strip; meansfor establishing a voltage gradient through the first of said titanatestrips, having a component directed parallel to the thickness dimensionof said strip; a plurality of electrodes disposed in'physical contactwith one of the faces of the second of said titanate strips, saidelectrodes lying transversely tothe length axis of said strip and beingspaced from one another; and electrical means for establishing apotential difference between said transversely lying electrodes.

9. In a bender type transducer: a bending unit including a pair ofelongated elements of electromechanically sensitive dielectric materialconnected to act conjointly, one of said elements having spacedelectrodes formed longitudinally on two opposite faces thereof; meansfor establishing a potential difference between said electrodes; aplurality of electrodes disposed in physical contact with one of thefaces of the second of said elongated elements, said electrodes lyingtransversely to the length axis of said element and being spaced fromone another; and a source of electrical potential for establishing apotential difference between said transversely lying electrodes.

l0. In a bender type transducer: a bending unit including a pair ofelongated elements of electromechanically sensitive dielectric materialconnected to act cenjointly, one of said elements having spacedelectrodes formed longitudinally on two opposite faces thereof; meansfor establishing a potential difference between said electrodes; aplurality of electrodes disposed in physical contact with both of thefaces of the second of said elongated elements, said latter electrodeslying transversely to the length axis of said element and being spacedfrom one another; and a source of electrical potential for establishinga potential difference between adjacent transversely lying electrodes.

11. In a bender type transducer: a plurality of elonf gated stripsjoined together in face to face relation, said strips including anelectrically conductive spacer member between them, said stripsincluding a first electrostrictive strip joined to one face of theconductive memher, a second electrostrictive strip joined to theopposite face of the member; a pair of spaced electrodes, each of whichis disposed in intimate physical contact with one of the oppposite majorfaces of the first of said electrostrictive strips; means forestablishing a potential difference between said electrodes; a pluralityof electrodes disposed in physical contact with one of the major facesof the second of said electrostrictive strips, said electrodes lyingtransversely to the length axis of the strip and being spaced from oneanother; and. means for establishing a potential difference between saidtransversely lying electrodes.

References Cited in the file of this patent I UNITED STATES PATENTS2,185,966 Pfanstiehl Jan. 2, 1940 2,497,108 Williams Feb. 14, 19502,540,187 Cherry Feb. 6, 1951 2,640,165 Howatt May 26, 1953 2,640,889Cherry June 2, 1953 FOREIGN PATENTS 678,825 Great Britain Sept. 10, 1952

